1. Field of the Invention
This invention relates to digital wireless communication systems, and more particularly to methods for improving transmit antenna weight tracking using channel correlations in a wireless communication system.
2. Description of Related Art
Digital wireless systems presently being deployed are commonly seen to be forward link (i.e., base station (“BS”)-to-mobile station (“MS”)) capacity limited. This is mostly due to asymmetric traffic scenarios: most data applications, such as web browsing, are envisioned to be sending relatively large quantities of data to the mobile user on the forward link, while the reverse link may contain only small control packets (i.e., a new IP address to download) so that the forward data rate is much greater than the reverse data rate.
Forward capacity can be increased with an array of antennae in a base station whose individual carrier amplitudes and phases can be adjusted based on feedback from a MS. In particular, a transmit adaptive antenna (TxAA) algorithm will typically transmit the same MS-specific waveform on multiple antennae, applying adaptive complex weights to the baseband signal applied to each antenna. To allow the MS to perform coherent demodulation, a dedicated pilot channel is typically transmitted in the same manner as the data.
While receive adaptive antenna (RxAA) algorithms at the BS are fairly straight forward, TxAA algorithms are not. The mere definition of an “optimal” TxAA algorithm is not unambiguous as the optimization of one mobile's forward link can degrade another's, leading to complex tradeoffs which are not part of the RxAA problem.
Most TxAA algorithms will require some a priori knowledge of the transmission channel from the BS to the MS. Given this knowledge and a defined “optimality” criterion, the BS can determine the TxAA weights. The difficulty is that, although the MS can measure the channel, it is the BS which needs the measured information to adjust its transmit weights. Also, there can be some additional complexity in that the MS must generally measure the channel of each transmit antenna separately, in addition to measuring the channel of the overall transmit (Tx) weight-adjusted signal. The latter measurement is required for the demodulation of the signal.
A number of methods for implementing TxAA algorithms have been proposed. According to some of these proposals, a few bits are allocated to the MS to encode the channel gain and phase. See, for example, Thomas Derryberry, Balaji Raghothaman (Nokia) “Transmit Adaptive Arrays without User Specific Pilot”, document # C30-19990817-030, submitted to 3GPP2 August 1999; Mark Harrison (Motorola) “Tx AA Parameter Recommendations”, document #C30-19990914-010, submitted to 3GPP2, Tokyo Japan, September 1999; and Mark Harrison, Kiran Kuchi (Motorola) “Open and closed loop transmit diversity at high data rates on 2 and 4 elements”, document #C30-19990817-17, submitted to 3GPP2 August 1999. These methods do not allow the desired antenna weights to be precisely determined because the channel state must be distorted in order to be fed back to the BS with a low bit rate.
Typically, the academic literature has assumed that the full channel information is available at the transmitter, which is not a practical assumption. See, e.g., Jen-Wei Liang, Arogyaswami Paulraj “Forward link antenna diversity using feedback for indoor communication systems” Proceedings, 1995 International Conference on Acoustics, Speech, and Signal Processing, May 1995; Farrokh Rashid-Farrokhi, K. J. Ray Liu, Leandros Tassiulas “Transmit beamforming and power control for cellular wireless systems” IEEE Journal on Selected Areas in Communications, Vol. 16, No. 8, October 1998. There have been several submissions to the TIA standardization body for TxAA algorithms with channel feedback from MS to BS, some of which are referenced above. All of these submissions require that the MS measures a primary and secondary pilot transmitted from the primary and secondary antennae. Some submissions have shown that 4 antenna transmission can yield improved performance, however, with no recognition of the increased complexity at the MS.
The use of a few bits of channel feedback leads to substantial degradation relative to possible performance, because the feedback requires low bit rate quantization. Also, the MS must individually measure and transmit information for each antenna; the MS requires extra hardware to perform these functions. Furthermore, the above-described methods do not gracefully grow to accommodate an increase in the number of antennae.
The above-mentioned systems are illustrated in FIGS. 1–3, which illustrate a CDMA system. FIG. 1 shows a transmitter 10 with two antennae, a first antenna (antenna #0) 12 and a second antenna 14 (antenna #1). As is shown, different common pilot signals are associated with different antennae. For each user, such as user #0 and user #1, the transmitter 10 includes an adder, e.g., the adders, 16 and 18 respectively, that adds together a dedicated pilot signal for the user and the forward traffic for that user. For user #0, multipliers 20 and 22 multiply the summed signal with complex weights for the first and second antennae 12 and 14, respectively. For each antenna, the weighted user signals are summed by adders 24 and 26 and the result is added by adders 28 and 30 to the pilot signal for that antenna. It should be noted that FIG. 1 represents a complex baseband equivalent, as no RF modulation stage is shown.
FIGS. 2 and 3 show possible embodiments for receivers that may be used to receive signals transmitted by the transmitter 10. As shown in FIG. 2, a received signal is divided into three components: one signal corresponding to the dedicated channel, one signal corresponding to the common pilot for the first antenna and one signal corresponding to the common pilot for the second antenna. This division is accomplished by multipliers 32, 34 and 36 and accumulators 38, 40 and 42. A more hardware efficient embodiment is shown in FIG. 3, which employs multiplexer 44 and demultiplexer 46 to alternately select between the signals for the different transmit antennae. The time-multiplexed processing used by the embodiment of FIG. 3 saves hardware at the expense of 3 dB loss of precision for each channel. This tradeoff may be acceptable when a low bit rate, low precision channel reporting is used by the mobile to report these channel estimates to the BS.
The channel estimate attained by the mobile is coded into a low bit representation. The bit rates mentioned in the above cited references are 1, 2 or 4 bits (1b phase, 2b phase, or 3b phase+1b amplitude). So, for a forward channel vector c, the mobile generates the estimate ĉ, which is then quantized to produce the feedback estimate {circumflex over (ĉ)} .
Note that the MS channel estimation hardware of FIGS. 2 and 3 can be used for an arbitrarily large number of Tx antennae, provided that there are unique pilot codes for each antenna and that the MS has information regarding these codes.
Finally, the proposed systems have the MS report the channel estimate based on 1 “path”. In the presence of resolvable multi-path due to delayed reflections of the transmitted waveform, particularly for CDMA, there may be more than one path that is usable to the MS. In order to report the channel for N such paths, the MS-to-BS feedback rate would be required to increase N-fold, and the number of such paths would somehow need to be communicated to the BS. This solution is not practical however, and instead, the MS reports the channel estimate for only the strongest path. This solution discards some useful channel characteristics which could further increase performance under these circumstances.
The algorithm employed by the BS to utilize the received channel information would most likely be a simple matched transmission weighting. That is, the forward weights chosen would be the conjugate of the forward vector channel, so that the weights are determined as follows:w={circumflex over (ĉ)}.
This formulation attempts to maximize signal power to the mobile without regard to the locations of other mobiles. It does not steer nulls to the other mobiles. The channel estimate from a given mobile could be used to determine transmission nulls of other mobiles, but the coarseness of the channel estimate (no greater than 4 bits) makes this approach ineffective.
Schemes such as that shown in FIGS. 1–3 have significant disadvantages. In particular, as previously mentioned, the use of a few bits of channel feedback requires quantization, which leads to substantial degradation relative to the possible performance. Also, because an MS unit must individually measure and transmit information for each antenna, the MS requires extra hardware to perform these functions. Furthermore, the above-described methods do not grow gracefully in application to more antennae.
One method used by the present invention in implementing a TxAA algorithm (referred to as the “Adaptive Antenna Method and Apparatus”) that overcomes these disadvantages is described in the above-incorporated U.S. Provisional Application No. 60/278,501, U.S. application Ser. No. 09/632,081 and hereinbelow. In accordance with this method, a transmitter includes hardware and/or software for adaptively updating weights for a plurality of antennae. In particular, according to the present invention, the transmitter generates a pair of test weight vectors, each vector comprising a plurality of complex entries, with each entry corresponding to a different one of a plurality of antennae. The first complex weight vector is equal to
            w      base        +          β      ·      v                              w        base            +              β        ·        v                and the second complex weight vector is equal to
                    w        base            -              β        ·        v                                            w          base                -                  β          ·          v                            ,where v is a test perturbation vector comprising a plurality of complex entries, with each entry corresponding to a different one of a plurality of antennae, β is an algorithm constant and wbase is a vector that is updated based on feedback received from a receiver. The first and second complex weight vectors are applied to a dedicated pilot signal during alternate time intervals. During each time interval, the average of the first and second complex weight vectors is applied to the data traffic transmitted by the transmitter.
A receiver (e.g., a mobile station or (“MS”)) alternately receives the pilot signal as multiplied by the first and second weight vectors as described above. The mobile station determines which of the weighted pilot signals resulted in a stronger signal received at the mobile and, based upon this determination, transmits feedback information. The transmitter receives the feedback and updates the first and second weights accordingly. In particular, if the first weight resulted in a stronger signal, wbase is updated to become
            w      base        +          β      ·      v                              w        base            +              β        ·        v                (the previous first weight) whereas if the second weight resulted in a stronger signal, w is updated to become
            w      base        -          β      ·      v                              w        base            -              β        ·        v                (the previous second weight). The above-described process is repeated with the new wbase. New test vectors v are generated and applied after each MS channel measurement is made.
A disadvantage of the adaptive antenna method and apparatus and the other TxAA techniques described above with reference to FIGS. 1–3 is that these techniques may track weight vectors at a relatively slow rate. These techniques provide no mechanism for using partial knowledge of the channel, such as the channel gain vector autocorrelation. Thus, a need exists for a method and apparatus for improving transmit antenna weight tracking. Such a method should utilize existing TxAA techniques to improve the performance of a wireless communication system. The inventive method should not degrade performance due to quantization from utilizing a few feedback bits. In addition, the method and apparatus should be able to accommodate multiple antennas.
The present invention provides such a method and apparatus for improving transmit antenna weight tracking.